Rechargeable batteries are used in a variety of industrial and commercial applications such as fork lifts, golf carts, uninterruptable power supplies, and electric vehicles. Rechargeable lead-acid batteries are a useful power source for starter motors for internal combustion engines. However, their low energy density (about 30 Wh/kg) and their inability to reject heat adequately, makes them an impractical power source for an electric vehicles (EV), hybrid electric vehicles (HEV) and 2-3 wheel scooters/motorcycles. Electric vehicles using lead-acid batteries have a short range before requiring recharge, require about 6 to 12 hours to recharge and contain toxic materials. In addition, electric vehicles using lead-acid batteries have sluggish acceleration, poor tolerance to deep discharge, and a battery lifetime of only about 20,000 miles.
Nickel-metal hydride batteries (“Ni—MH batteries”) are far superior to lead-acid batteries, and Ni—MH batteries are the ideal battery available for electric vehicles, hybrid vehicles and other forms of vehicular propulsion. For example, Ni—MH batteries, such as those described in U.S. Pat. No. 5,277,999, the disclosure of which is incorporated herein by reference, have a much higher energy density than lead-acid batteries, can power an electric vehicle over 250 miles before requiring recharge, can be recharged in 15 minutes, and contain no toxic materials.
Extensive research has been conducted in the past into improving the electrochemical aspects of the power and charge capacity of Ni—MH batteries, which is discussed in detail in U.S. Pat. Nos. 5,096,667, 5,104,617, 5,238,756 and 5,277,999, the contents of which are all incorporated by reference herein.
Until recently the mechanical and thermal aspects of the performance of Ni—MH batteries have been neglected. For example, in electric vehicles and in hybrid vehicles, the weight of the batteries is a significant factor. For this reason, reducing the weight of individual batteries is a significant consideration in designing batteries for electric and hybrid vehicles. Battery weight should be reduced while still affording the necessary mechanical requirements of the battery (i.e. ease of transport, ruggedness, structural integrity, etc.).
Electric vehicle and hybrid vehicle applications introduce a critical requirement for thermal management. Individual electrochemical cells are placed together in close proximity and many cells are electrically coupled together. Therefore, since there is an inherent tendency to generate significant heat during charge and discharge, a workable battery design for electric and hybrid vehicles is judged by whether or not the generated heat is sufficiently controlled. Sources of heat are primarily threefold. First, ambient heat due to the operation of the vehicle in hot climates. Second, resistive or I2R heating on charge and discharge, where I represents the current flowing into or out of the battery and R is the resistance of the battery. Third, a tremendous amount of heat is generated during overcharge due to gas recombination.
In the past, Ni—MH batteries employed metal battery cases made of such materials as aluminum, nickel and stainless steel. These cases provided the batteries with an efficient path for removal of internal heat via the excellent thermal conductivity of these metals. Therefore, it was not very difficult to provide the batteries and modules (bundles) of such batteries with effective thermal cooling even when only a portion of the case was exposed to the cooling medium.
Today, in an effort to reduce the weight of such batteries (particularly in vehicle applications), the cases for these types of batteries (and modules) are made of plastic. The specific plastics and/or blends/alloys that have been used up to now are chosen for their physical properties, dielectric properties and chemical resistance to the environment and the electrochemical cells internal chemistry. Unfortunately, these plastics generally have relatively low thermal conductivity, and as such their use generally places severe limitations on the ability of the batteries to be cooled efficiently, and therefore more elaborate systems are needed to provide both the structural integrity and thermal management of the batteries.
What is needed in the art is batteries made with lightweight cases that are formed from lightweight plastic blends that have the mechanical, dielectric and chemical resistance properties required for such battery cases and additionally have enhanced thermal conductivity. These batteries will make it easier to thermally manage the batteries, battery modules and battery packs (via cooling with gas and/or liquids coolants) by providing for ease and uniformity of cooling and flexibility of cooling design options.